Structures (e.g. river bridges) may be in and/or on beds underlying flowing water. Some examples of flowing water are flood flows and/or tidal flows, for instance water flowing in bays, estuaries, inlets, locks, rivers, seas, straits, sounds or weirs. Stability of a structure (e.g. a bridge), is threatened when flow of water scours or otherwise erodes the bed in the neighbourhood of the bridge's substructure, e.g. at a bridge's pier. The erosion may lead to collapse of a bridge, etc.
Inspections of e.g. bridges may be made by divers during low flow conditions of rivers. But, inspection of a bridge at a threatening times of flood may amount only to cursory visual examination of the bridge's superstructure, i.e. at location(s) above the surface of a river. Thus, it is possible not to detect erosion of a river bed in the neighbourhood of a bridge or other structure, during times of expected flooding or other turbulent conditions. Monitoring of e.g. a bridge during those times may be obscured by air entrainment, debris, sediment load, etc. comprised by a river.